The units for the production of carbon monoxide and hydrogen can be separated into two parts:                generation of synthesis gas (mixture comprising H2, CO, CH4, CO2, Ar and N2 essentially). Among the various industrial routes for the production of synthesis gas, that based on coal gasification appears to become increasingly more prevalent, in particular in countries rich in coal deposits, such as China. The design of this unit, which comprises a reactor for the gasification of coal with oxygen and steam, is based on the required production of CO and the required production of hydrogen.        purification of the synthesis gas. The following are found:        a unit for scrubbing with a liquid solvent, in order to remove the larger part of the acid gases present in the synthesis gas,        a unit for purification over a bed of adsorbents,        a unit for separation by the cryogenic route, “cold box”, for the production of CO.        
Generally, the synthesis gas comprises a mixture at high pressure (generally between 30 and 60 bar) and is very rich in CO (approximately 50 mol %). Another advantage of the coal gasification process is the low content of impurities (CH4, argon and nitrogen) present in the synthesis gas at the inlet of the cold box for the production of pure CO.
This makes it possible to envisage a relatively simplified flowchart for the cold box, the cryogenic separation being limited to a separation between CO and hydrogen. The contents of inert substances in the synthesis gas are compatible with the CO purity required by the client in the majority of cases.
This flowchart does not comprise a cycle dedicated to separation.
The hydrogen separated from the CO is required at high pressure in order to be able to make economic use of it, either in a PSA or in a unit for the synthesis of methanol or other.
A portion of the separation energy of said cold box is provided by free expansion between the synthesis gas and the pure CO produced at low pressure but, in the majority of cases, this free expansion is not sufficient to complete the refrigeration balance of the unit. A supply of liquid nitrogen is necessary in order to keep the cold box cold and to complete the refrigeration balance.
The synthesis gas at a pressure generally of between 30 and 60 bar coming from a pretreatment unit (CO2 and MeOH separation) is cooled in the main exchange line and partially condensed before feeding a one-stage partial condensation separator pot. The hydrogen-rich vapor is in the majority of cases conveyed to an MeOH unit or to a PSA after reheating in the exchange line. The bottom liquid is conveyed to a medium-pressure stripping column (about 14 bar) after expansion. The top vapor, known as flash gas, exits from the cold box after reheating and is conveyed as fuel or recycled material to the system via a compressor.
A stream withdrawn from the column at a level above the vessel bottom is subcooled to a certain temperature level, expanded, conveyed to a thermosiphon pot and then evaporated in the exchange line before being conveyed to the suction port of the CO compressor.
The bottom liquid from the stripping column is subcooled in the exchange line to a temperature level less cold than that of the stream mentioned above before being expanded, evaporated and reheated in the main exchange line and finally conveyed to an intermediate stage of the CO compressor.
A third stream can be withdrawn at another level of the column (a level above the bottom), subcooled to a temperature level different from the two preceding levels, expanded, evaporated and reheated in the exchange line.
The CO compressor makes it possible to compress the CO produced (which is the sum of all the liquid withdrawals from the stripping column) to the pressure required by the downstream unit (acetic acid or other).
The advantage of subcooling the flows of carbon monoxide to different temperature levels is to reduce the KS and the heat load and thus the capital cost of the main exchange line. The medium-pressure flow(s) is (are) subcooled to (a) temperature level(s) which is (are) less cold than if they were all mixed.
This makes it possible to reduce the electrical energy of the CO compressor 33, 35, 37. Less energy is consumed for the subcooling of the medium-pressure levels. Each of the CO-rich flows is subcooled only to a certain temperature level which prevents the creation of gas after expansion, which makes it possible to avoid having to install two-phase (liquid-gas) introduction pots for the main exchange line.
This makes it possible to have a low-pressure fluid rich in carbon monoxide (lowest pressure level at the thermosiphon pot 27) which can comprise more hydrogen than the bottom of the stripping column and thus makes it possible to have a low evaporation temperature for an identical pressure level and thus makes it possible to further cool the synthesis gas (for an identical ΔT) and to improve the CO output of the unit. Alternatively, at an identical output, this makes it possible to reduce the electrical energy of the compressor as it is possible to increase the suction pressure of the compressor.